So much for Newt Gingrich's "southern strategy." With primary losses in both Alabama and Mississippi on Tuesday, the former House speaker seemingly lost his main argument for staying in the race: He can no longer claim to be the preeminent conservative alternative to Mitt Romney, a title that now clearly belongs to Rick Santorum, who won both southern contests. Gingrich "is toast," says Ed Kilgore at The New Republic. Observers say Gingrich has little-to-zero chance of winning a majority of convention delegates, and conservatives are calling on him to drop out so that the base can unite around Santorum. But Gingrich vows he will fight on, leaving some in the GOP scratching their heads. Here, 4 theories why Gingrich is digging in his heels:

1. He thinks he can win a brokered convention
Gingrich knows he has no chance of winning the 1,144 delegates needed to secure the nomination, says Byron York at The Washington Examiner. But he thinks he can help prevent Romney from gaining a majority of delegates, thereby triggering a "brokered GOP convention at which Gingrich would emerge as the eventual nominee." Gingrich has been hinting at such a scenario on the campaign trail, talking about a "political process beyond" the primaries. He also consistently brings up the example of President Warren G. Harding, who in 1920 came in sixth in the primary contest, but went on to win the nomination after the convention.

2. He hopes to be rewarded for helping Romney out
Gingrich is the "biggest moving piece in the race," says Jonathan Chait at New York. His "continued presence" is "highly valuable to Romney" and the GOP establishment, because Gingrich will split the conservative vote with Santorum. After all, Gingrich is the "only reason" the contests in Alabama and Mississippi "were even remotely close." It's possible that "he wants to leverage that asset. You never really can predict anything with Gingrich." Some observers have noted that Gingrich may be eyeing a cabinet post.

3. He wants to be Santorum's VP
The Gingrich campaign "likes the idea" of Santorum and Gingrich "running on the same ticket," says Jon Ward at The Huffington Post. Gingrich believes they would "make a powerful team against Barack Obama," according an unnamed senior adviser in the Gingrich camp. Maybe the Gingrich campaign is waiting for a VP offer to induce Gingrich to drop out.

4. He has personal reasons 
Gingrich is sticking around "to settle some scores" with his adversaries, Charles Krauthammer tells Fox News. Actually, Gingrich is just too proud to give up, says Erick Erickson at RedState. He could be the race's "king maker," but instead he's limping his way towards a "sad end to a brilliant legacy." Above all, Gingrich sees himself as a "transformative figure" and the "definer of civilization," says John J. Pitney Jr. at The National Review. Boosting Santorum's odds against Romney "may seem like small potatoes" to him.